Optical discs are a subject of research in the information recording field. They have a wide range of applications, from industrial use to consumer use, because they enable non-contact recording and reproduction, because they can accommodate large files at low cost, and because reproduction-only, writable, or rewritable media can be selected according to the application.
The capacity of optical discs has been increased by reducing the size of the information stored on the tracks, by using laser beams having shorter wavelengths as light sources for recording and reproducing, by using objective lenses with higher numerical apertures, and by reducing the size of the focused light spot on the focal plane.
For example, in a CD (compact disc), the thickness of the disc substrate used as a light transmitting layer is about 1.2 mm, the wavelength of the laser beam is about 780 nm, the numerical aperture (NA) of the objective lens is 0.45, and the disc capacity is 650 MB. In a DVD (digital versatile disc), the thickness of the disc substrate used as a light transmitting layer is about 0.6 mm, the wavelength of the laser beam is about 650 nm, the numerical aperture of the object lens is 0.6, and the disc capacity is 4.7 GB. For a DVD, two disc substrates having thicknesses of about 0.6 mm each are laminated together, to obtain a disc having a thickness of about 1.2 mm.
A BD (Blu-ray disc) disc having a still higher density uses an optical disc having a protection layer as thin as 0.1 mm as the light transmitting layer on the optical recording layer. With a laser beam wavelength of about 405 nm and a numerical aperture of 0.85, capacities in excess of 23 GB are obtained.
Thus, as optical disc capacities have increased, various optical disc standards have been established, including CD, DVD, and BD. Optical disc recording and reproducing devices are generally compatible with a plurality of different optical disc standards.
For example, some DVD recording and reproducing devices can record and reproduce information not only on a DVD but also on a CD. Some BD recording and reproducing devices can record and reproduce information on BDs, DVDs, and CDs. These compatible recording and reproducing devices are extremely convenient because they allow the user to record and reproduce information on optical discs in the user's possession that conform to old standards, and they have played an important role in smoothing the introduction of new standards.
Although discs have been manufactured according to each of the different standards, to increase the user's convenience, a multilayer optical disc having information layers conforming to a plurality of different optical disc standards has been developed (for example, patent document 1).    Patent document 1: Japanese Unexamined Patent Application Publication No. 2004-95005